At present, the application range of liquid crystal compounds become increasingly widened, including their application in various types of displays, electro-optical devices, sensors and so on. There are numerous species of liquid crystal compounds that are used in the field of display. Among them, nematic liquid crystals are most widely used, and for example, they have been used in passive matrix-type TN, STN matrix display and a system having TFT active matrix.
The thin film transistor liquid crystal display (TFT-LCD) has an enormous market and gradually become mature in recent years. However, the requirements on display technologies are continuously increased, especially in terms of realizing the fast response, and decreasing the driving voltage to lower the power consumption. As one of optoelectronic materials important for the liquid crystal display, the liquid crystal material plays an important role in improvement of the performances of the liquid crystal display.
The liquid crystal materials need to have good chemical and thermal stabilities, and have stabilities to electric and electromagnetic radiation. However, in addition to these, the liquid crystal materials used in TFT-LCD also should have a wide temperature range of the nematic phase, a suitable birefringence anisotropy, an extremely high resistivity, a good anti-UV performance, a high charge retention, a low vapor pressure, and so on.
For dynamic image display application, for example, liquid crystal display television, the liquid crystal is required to have a quite fast response speed and thus a low rotary viscosity γ1, in order to achieve high quality display and eliminate image ghosting and trailing. Furthermore, in order to lower the energy consumption of the device, the driving voltage of the liquid crystal is expected to be as low as possible. Accordingly, increase of the dielectric anisotropy Δ∈ of the liquid crystal is of great importance for mixed liquid crystals.
Numerous studies show that once a linking group difluoromethyleneoxy (—CF2O—) is introduced in a liquid crystal, the rotary viscosity γ1 of the liquid crystal would be lowered. Moreover, due to the contribution of the difluoromethyleneoxy bridge (—CF2O—) to dipole moment, the dipole moment of fluorine atom in the terminal group is improved to some degree, so that the dielectric anisotropy Δ∈ of the liquid crystal molecule is increased. Merk of Germany and Chisso Corporation of Japanese have disclosed some liquid crystal compounds having difluoromethyleneoxy (—CF2O—) as a linking group and substituted with different groups (e.g. CN1717468A, CN101143808A, and CN101157862A etc). However, the introduction of the group —CF2O— will greatly lower the clearing point of the liquid crystal. The decrease in the clearing point caused by the group —CF2O— needs to be compensated by adding compounds having a high viscosity and a high clearing point in the formulation of a liquid crystal mixture, which restricts the increase space in response speed of the liquid crystal mixture.
As a core functional material in liquid crystal display devices, the liquid crystal material is required to have a wide range of performance parameters, in order to meet the requirements of various performance parameters imposed by the liquid crystal display device, and accommodate for the process requirements of the liquid crystal display device. However, any single one of monomer liquid crystal materials cannot meet all of the requirements, and thus it is expected to synthesize monomer liquid crystals having distinct performances, in order to meet the property requirements of the LCD device through formulation and mixing.